1. Field of the Invention
The present invention relates to a surface illuminator utilizing an array of discrete light sources and a liquid crystal display having the same.
2. Description of the Related Art
A liquid crystal display is provided with a surface illuminator on a front side or back side of a liquid crystal display panel thereof. Backlight units which are surface illuminators disposed on the back side of a panel include side light (edge light) types including a light source disposed along a side edge of a light guide for guiding light and direct types including a light source disposed directly under a liquid crystal display panel. While cold-cathode tubes are used as light sources in general, it is not preferable to use a cold-cathode tube that utilizes mercury under the recent circumstance in which environmental problems are taken seriously. For this reason, various light sources such as mercuryless fluorescent tubes and LEDs (light-emitting diodes) have been developed as light sources to replace cold-cathode tubes, and LEDs are regarded most promising as next generation light sources.
When LEDs are used as a light source of a side light type backlight unit, the light source may be configured by arranging a plurality of white LEDs or arranging a plurality of sets of LEDs, each set consisting of monochromatic LEDs emitting light in different colors (e.g., red (R), green (G), and blue (B)). A white LED is a combination of a yellow-emitting fluorescent body and a blue-emitting LED, and it has relatively small variation in the color of emission. Backlight units utilizing a combination of R, G, and B monochromatic LEDs are attracting keen attention for their capability of achieving a vast range of color reproducibility (e.g., the adobe RGB range) which is not achievable with white LEDs.
A proposal has been made on a system (sub light guide system) involving a light guide region which is provided for mixing the colors of beams of light emitted by an array of discrete light sources and which is not used as a display area (see Non-Patent Document 1). However, there is a problem in that the system as a whole results in very low utilization of light because light enters a sub light guide from LEDs and enters a main light guide from the sub light guide at low efficiency. Since the low utilization of light necessitates an increase in the power supplied and consequently necessitates countermeasures against heat, a problem also arises in that the size of a device is increased to accommodate radiation fins. As a solution to those problems, a backlight unit has been proposed, in which an air region having a predetermined thickness is provided between a light guide and a diffusing plate to improve mixing of emission colors. FIG. 30 schematically shows a light guide and LED modules used in the proposed backlight unit. As shown in FIG. 30, LED modules 115 are provided on both of side surfaces (light entrance surfaces) 112T and 112B along the longer sides of a rectangular light-emitting surface 116 of a light guide 110 in the form of a thin plate. Both of side surfaces along the shorter sides of the light guide 110 are constituted by reflective surfaces 113L and 113R to allow high utilization of light from the LED modules 115.
The number of LEDs 77 in each color among the LED modules 115 is determined by the setting of white balance. Normally, one set of LEDs is constituted by a combination of one each B (blue) emission LED 77B, R (red) emission LED 77R, and G (green) emission LED 77G. By arranging the LEDs 77 in each color at equal intervals, the colors of the individual LEDs 77 are visually perceived at substantially the same distance from a light entrance surface 112 regardless of the types of the LEDs 77.
Therefore, the LED modules 115 include LED sets (hereinafter also referred to as “GRB” sets as occasion demands) 100 located at side ends of the reflective surface 113L, the LED sets being a series of LED sets starting at the ends of the reflective surface 113L and each consisting of a G (green) emission LED 77G, an R (red) emission LED 77R, and a B (blue) emission LED 77B provided in the order listed. A plurality of the LED modules 115 are provided in series from the left ends of the light entrance surfaces 112T and 112B of the light guide 110, “GRB” sets 100 serving as unit light sources, the width of the “GRB” sets constituting the pitch of the modules.
When such a configuration is employed, at an arbitrary point inside the light exit surface 116, three beams of light from a G-emission LED 77G, an R-emission LED 77R, and a B-emission LED 77B in the neighborhood of the arbitrary point can be mixed to generate a beam of light in a desired color.
Let us now discuss an arbitrary point which is located, for example, in the neighborhood of the top of the reflective surface 113L along the left shorter side of the light exit surface 116. Light arriving at this point includes not only direct light from the “GRB” set 100 at the left end of the light entrance surface 112T but also light from a “BRG” set 101 which is a mirror image generated as a result of reflection of the light from the “GRB” set 100 at the reflective surface 113L. The result is equivalent to arranging a set of six LEDs, i.e., B, R, G, G, R, and B LEDs in the order listed from the left side of the figure in the neighborhood of the arbitrary point near the top of the reflective surface 113L. Beams of light from a four-LED set, i.e., an “RGGR” set that is located closer to the arbitrary point are mixed with each other at relatively high intensity, and there will be no “B” beam at all. As a result, mixed white that lacks a blue component is generated ay the arbitrary point, and a problem therefore arises in that a yellowish color irregularity is liable to occur in the neighborhood of the top end of the reflective surface 113L. Such a color irregularity can occur also at the bottom end of the reflective surface 113L and the top and bottom ends of the reflective surface 113R on the right when the same LED arrangement is employed. The same problem occurs even if the LED sets 100 have a “GBR” pattern instead of the “GRB” pattern.
A method of avoiding the problem is to exclude regions having such a color irregularity from the display area, but the method is not preferable in that the picture frame region of the display will be consequently expanded by about 5 cm in the vertical or horizontal direction of the display. In the case of a large LED backlight employing 100 or more LEDs, there are variations in the emitting efficiency and emission wavelength of the LEDs, and the variation will result in irregularities in the vicinity of the picture frame.
Patent Document 1: JP-A-2003-215349
Patent Document 2: JP-A-2004-95390
Non-Patent Document 1: Nikkei Electronics No. 844 pp. 126-127, Mar. 31, 2003